Polyacetal polymers, which are commonly referred to as polyoxymethylene polymers, have become established as exceptionally useful engineering materials in a variety of applications. For instance, because polyoxymethylene polymers have excellent mechanical properties, fatigue resistance, abrasion resistance, chemical resistance, and moldability, they are widely used in constructing polymer articles, such as articles for use in the automotive industry and the electrical industry.
The mechanical properties of polyoxymethylene molding compositions are the reason for their use in numerous applications. To improve their properties, polyoxymethylene polymers are often provided with additives to adapt the properties for a specific application, for example by using reinforcing fibers or tribological modifiers. For instance, polyoxymethylene polymers have been combined with a tribological modifier for producing polymer compositions well suited for use in tribological applications where the polymer article is in moving contact with other articles, such as metal articles, plastic articles, and the like. These tribological applications can include embodiments where the polymer composition is formed into gear wheels, pulleys, sliding elements, and the like. The addition of a tribological modifier can provide a composition with a reduced coefficient of friction and low wear.
In the past, high molecular weight polyolefins have been used to improve the wear resistance of polyoxymethylene resins. For instance, U.S. Pat. No. 5,482,987, which is incorporated herein by reference in its entirety, discloses a self-lubricating, low wear composition containing a polyoxymethylene and a lubricating system comprising a high molecular weight polyethylene. U.S. Pat. No. 5,641,824, which is incorporated herein by reference in its entirety, discloses a self-lubricating melt blend of a polyoxymethylene and an ultra-high molecular weight polyethylene.
In addition to high molecular weight polyolefins, numerous other tribological modifiers have been proposed in the past. For instance, other tribological modifiers that have been used in the past include silicones such as silicone oil, polysiloxane, waxes, and the like. Each tribological modifier can display different properties depending upon the particular application. Thus, the use of tribological modifiers in particular applications has been somewhat unpredictable.
In certain applications, in addition to reducing the coefficient of friction and reducing wear, it is desirable that the polymer composition also have aesthetic appeal. For instance, although tribological modifiers can produce compositions having low friction characteristics, the triological modifiers can have a tendency to create surface imperfections on molded articles made from the compositions and/or may adversely interfere with colorants that may be added to the composition. Thus, finding a tribological modifier that not only reduces the coefficient of friction but also that does not interfere with the surface characteristics of molded articles made from the composition has been problematic. In this regard, the present disclosure is directed to polymer compositions that have low friction characteristics, especially when tested against metals such as aluminum, and that are compatible with colorants without producing significant amounts of surface imperfections during molding.